APPLICATIONS  |  CIS SOLAR CELLS

RESULTS          ABSOLUTE CALIBRATION          GLOBAL IMAGING          RELATED PRODUCTS


 

Within the second generation solar cells, copper indium disulfide (CuInS2 or CIS) is one of the most promising material. It has been under the scope of scientists in the field of photovoltaics since the early 90s, when it already exhibited an efficiency exceeding 10% [1]. Its high absorption coefficient, direct band gap (1.52 eV) [2] and nontoxicity make it an ideal candidate for both thin films and quantum dot-sensitized solar cells. However, CIS efficiency seems to have reached a plateau. To keep improving the next generation of CIS cells and go beyond this limitation, a clear understanding of the impact of the fabrication methods on the cell’s properties is necessary.

With this in mind, researchers at IRDEP (Institute of Research and Development on Photovoltaic Energy) characterized multicrystalline CuInS2 cells using spectrally and spatially resolved photoluminescence (PL) imaging. The hyperspectral platform (IMA™) provides a 2 nm spectral resolution and a spatial resolution below 2 μm. The device is uniformly excited by a 532 nm laser over the whole field of view under the microscope objective. FIG. 1 shows the integrated PL map of the device and FIG. 2 presents PL spectra of selected regions on the studied area [3]. The global imaging modality provides rapid highlights of spatial inhomogeneities. With this technique, researchers are able to spatially monitor several properties. Indeed, PL maxima offers detailed maps of both the bandgap and the fluctuations of the quasi-fermi level splitting [4]. With the help of their patented spectral and photometric absolute calibration method (see section below), researchers at IRDEP can extract maps of optoelectronics properties of their devices (e.g. : EQE, Voc, etc.)

While a confocal microscope coupled to a spectrometer could provide similar data, we can show that this alternative time-consuming technique is impractical (FIG. 3). In this case, the 532 nm laser is focused onto the cell front contact and the PL cartography is obtained one point at a time. Comparing the acquisition time of the two methods, global hyperspectral and confocal microscopy, we see that a 150x150 µm² map at 107 W/m² takes only 8 minutes to acquire with the former, but would take hundreds of hours with a confocal microscope [3].

[1] Scheer R., Walter T., Schock H. W., Fearheiley M. L., Lewerenz H. J., CuInS2 based thin film solar cell with 10.2% efficiency, Applied Physics Letters, 63, (1993).
[2] Suriakarthick R. et al., Photochemically deposited and post annealed copper indium disulfide thin films, Superlattices and Microstructures, (2014).
[3] Delamarre A. et al., Characterisation of solar cells using hyperspectral imager, IRDEP.
[4] Delamarre A. , Paire M., Guillemoles J.-F.  and Lombez L., Quantitative luminescence mapping of Cu(In,Ga)Se2 thin-film solar cells, Progress in Photovoltaics, 10, 1002, (2014).

SPECTRAL AND PHOTOMETRIC ABSOLUTE CALIBRATION

Most if not all luminescence characterisation techniques provide data in arbitrary units. A deep interpretation of such results is often limited by this lack of information. With this in mind, researchers at IRDEP developed a powerful method for spectral and photometric calibration. With this technique, they are able to determine the absolute number of photons of a given energy emitted from every point of the surface of their sample. By performing this calibration, researchers can further investigate Planck’s law and the reciprocity relations between a solar cell EQE and the EL emitted at a given voltage [1]. Hence, the absolute calibration of the hyperspectral data provides a direct way to extract spatial variations of several properties such as open circuit voltage (Voc), saturation currents and external quantum efficiency (EQE).

In order to perform an absolute calibration and measure the signal to get the number of photons, two steps are needed [2]. First, for each wavelength of the spectral region of interest, a relative calibration is achieved on a given area by coupling a calibrated halogen lamp to an integrating sphere. This setup, providing a spectrally and spatially homogeneous output, allows the correction of sensitivity fluctuations. Then, an absolute calibration is carried out for a given wavelength on a single point of the sample. To do so, the output of a fibered coupled laser is imaged and compared with the intensity measured with a power meter. Finally, combining the relative calibration of the whole sample and spectral range to the absolute calibration at a given wavelength and point,  the absolute calibration of the whole sample can be extrapolated for every wavelength of interest.


[1] Rau, U., Reciprocity relation between photovoltaic quantum efficiency and electroluminescent emission of solar cellsPhysical Review B 76, (2007).
[2] Delamarre A. , Paire M., Guillemoles J.-F.  and Lombez L., Quantitative luminescence mapping of Cu(In,Ga)Se2 thin-film solar cellsProgress in Photovoltaics, (2014).

GLOBAL IMAGING MODALITY

As previously stated, this hyperspectral platform allows the acquisition of the entire field of view under a microscope, wavelength after wavelength. Using a megapixel sensor, the acquisition of filtered images will provide spectral information from million of points at the surface of the sample. By design, this modality requires uniform illumination over the entire field of view. When compared to typical confocal PL setups where the excitation is done at only one point (~1 μm2), thus leaving the surrounding area at rest, global illumination avoids the recombination of carriers due to localized illumination. Indeed, the isopotential created when using global illumination prevents the above mentioned charge diffusion. In confocal setups, lateral diffusion of carriers towards the darker regions of a sample has the effect of reducing the PL signal so the excitation power needs to be increased considerably in order to observe PL signal. This high power density is far from what the PV material will ever experience in real conditions. In fact, the power density used in confocal microscopy usually reaches 104 suns, far from the operating conditions of a photovoltaic device, which is a serious complication for the interpretation of the results. Homogeneous illumination used for the global imaging modality allows carrying PL experiments in the range of 1 - 500 suns which is within the range of realistic operating mode of concentrated PV.

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IMAPL

From solar cells to advanced materials, our fast and all-in-one hyperspectral microscope IMA PL offers unmatched image and data quality.

IMAEL

Perfectly suited for the analysis of photovoltaic cells and semiconductors, IMA EL is a fast hyperspectral microscope for the characterization of materials by means of electroluminescence.

HYPERCUBE

The HyperCube™ will transform your microscope into a high resolution spectral imaging system, opening new research perspectives in biological imaging. Designed to fit commercial microscopes, cameras and a vast variety of excitation modules, The HyperCube™ gives access to the detailed composition of your sample.

TUNABLE LASER SOURCE

Our turn-key sources unite the flexibility of supercontinuum light sources to the incomparable out-of-band rejection of our optical filters, allowing easy and precise sample excitation or instrument calibration.